Temperature compensation circuit and method for position sensors

Abstract

A circuit and method to compensate for thermal effects on potentionmeter based sensors by presenting two input signals at different times to a comparator; the first utilizing one leg of the potentiometer and the second utilizing both legs. The comparator output will change states after receiving each input signal. The time periods between the receipt of the input signal and the outputs' change of state are stored. The quotient of the time periods cancels out most of the thermal effects on the potentiometer and is still a representative of the original potentiometer signal.

I claim: 
     
       1. In a position sensing device to sense the position of a movable portion of a mechanism, the position sensing device with a controller to store data from and to present control signals to a temperature compensation circuit comprising: a variable resistor with two end leads and a wiper arm;   a switching device connected across the two end leads of the variable resistor with the ability to short the two end leads upon command from a control signal from the controller to the device;   the wiper arm of the variable resistance device being mechanically connected to the movable portion of the mechanism;   a comparator with inverting and non-inverting inputs;   a voltage supply means, a fraction of which is connected to the non-inverting input to the comparator as a reference voltage;   a resistor connected between the wiper arm of the variable resistor and the inverting input to the comparator;   a capacitor with one lead connected to the wiper arm of the variable resistor and the other lead available for a step function control signal from the controller;   the comparator, changing output states when the voltage at its inverting input exceeds or drops below the reference voltage at the non-inverting input in response to the changing voltage on the capacitor as signaled by the controller;   the controller, storing the output curve of the comparator and providing a position signal representative of the position of the movable portion of the mechanism;   the position signal derived by comparing portions of the comprator's output curve generated in response to the step function control signals presented to the capacitor.   
     
     
       2. In a position sensing device to sense the position of a movable portion of a mechanism, the position sensing device with a controller to store data from and to present control signals to a temperature compensation circuit comprising: a variable resistor with two end leads and a wiper arm;   a first switching device, connected between one end of the variable resistor and ground with the ability to receive control signals from the controller;   a second switching device, connected between the other end of the variable resistor and ground with the ability to receive control signals from the controller;   a comparator with inverting and non-inverting inputs;   a voltage supply means, a fraction of which is connected to the non-inverting input to the comparator as a reference voltage;   a resistor connected between the wiper arm of the variable resistor and the inverting input to the comparator;   a capacitor with one lead connected to the wiper arm of the variable resistor and the other lead available for a step function control signal from the controller;   the comparator, changing output states when the voltage at its inverting input exceeds or drops below the reference voltage at the non-inverting input in response to the changing voltage on the capacitor as signaled by the controller;   the controller, storing the output curve of the comparator and providing a position signal representative of the position of the movable portion of the mechanism;   the position signal derived by comparing portions of the comparator's output curve generated in response to the step function control signals presented to the capacitor.   
     
     
       3. A method of temperature compensating a potentiometer based position sensing system, the potentiometer comprising a variable resistor with end leads and a wiper arm, where the wiper arm is connected to the movable portion of a mechanical device, with a discharging circuit, comparator means and a controller means, including the steps of: activating a first portion of the potentiometer to work with the charging circuit such that the resistance between the wiper and one of the two end leads of the potentiometer is utilized;   presenting a first control pulse from the controller to the charging circuit;   comparing the output of the charging circuit to a fixed threshold, forcing the comparator means to change states when the threshold is crossed;   storing the period between the first control pulse and the subsequent change of state of the comparator means;   activating a second portion of the potentiometer, that portion between the wiper arm and the other of the two end leads, to work with the charging circuit such that both the first portion and second portion are in parallel with each other with respect to the wiper arm;   presenting a second control pulse from the controller to the charging circuit;   comparing the output of the charging circuit to a fixed threshold, forcing the comparator means to change states when the threshold is crossed;   storing the time period between the second control pulse and the subsequent change of state of the comparator means;   using the quotient of the time periods as a representative measure of the motion of the movable portion of the mechanical device.   
     
     
       4. A method of temperature compensating a potentiometer based position sensing system, the potentiometer comprising a variable resistor with two end leads and a wiper arm where the wiper arm is connected to the movable portion of a mechanical device, with a charging circuit, comparator means and controller means, including the steps of: activating a first portion of the potentiometer to work with the charging circuit such that only the resistance between the wiper arm and one of the two end leads of the potentiometer is utilized;   presenting a first control pulse from the controller to the charging circuit;   comparing the output of the charging circuit to a fixed threshold, forcing the comparator means to change states when the threshold is crossed;   storing the period between the first control pulse and the subsequent change of state of the comparator means;   activating a second portion of the potentiometer, that portion between the wiper arm and the other of the two end leads, to work with the charging circuit such that only resistance between the wiper arm and the other of the two end leads of the potentiometer is utilized;   presenting a second control pulse from the controller to the charging circuit;   comparing the output of the charging circuit to a fixed threshold, forcing the comparator means to change states when the threshold is crossed;   storing the period between the second control pulse and the subsequent change of state of the comparator means;   using the quotient of the time periods as a representative measure of the motion of the movable portion of the mechanical device.